ci: improve CI stability with parallel builds and Valgrind

Major changes:
Run 4 build configurations in parallel with Valgrind on Linux
Add test failure detection verification (meta-test)
Port allocation per build to prevent conflicts in parallel runs
Add --coverage-only flag for dedicated coverage builds
Code quality:

Add TEST_GROUPS constant, CMOCKA patterns, helper functions
Organize ci-build.sh into sections

.gitignore

@@ -62,6 +62,8 @@ tests/unittests/unittests
 tests/unittests/unittests.log
 tests/unittests/unittests.trs
 test-suite.log
+test-files/
+test-logs/
 
 # valgrind output
 profval*
@@ -108,3 +110,7 @@ breaks
 *.zip
 *.log*
 coverage/
+*.gcno
+*.gcda
+*.gcov
+coverage.info